1. Field of the Invention
The present invention relates to an apparatus for detecting a flow rate, and more particularly to a flow rate sensor of an internal combustion engine, a flow rate sensor used in a fuel battery system or the like.
2. Description of Related Art
Conventionally, as an air flow rate sensor provided in an intake air passage of an internal combustion engine of a motor vehicle or the like, and measuring an amount of an intake air, a thermal type sensor comes to a mainstream because the thermal type sensor can directly detect a mass air amount. Recently, since an air flow rate sensor particularly manufactured by a semiconductor micromachining technique has a high speed response and can detect a back flow by utilizing a speed of the response, such an air flow rate sensor has been remarked.
A technique of the conventional thermal type air flow rate sensor using the semiconductor substrate mentioned above is disclosed, for example, in patent document 1 (JP-B2-3545637).
In this conventional embodiment, a flow rate detecting element using a tabular semiconductor substrate is accommodated in a support body having a concave portion, and is arranged in such a manner as to be approximately in parallel to a flow direction of the air flow and in such a manner that a surface of the flow rate detecting element has a predetermined surface position with a surface of the support body.
The prior art has the following problems. FIG. 14 shows a support body accommodating a flow rate detecting element of a thermal type flow meter described in the patent document 1, and FIGS. 15 and 16 show a cross section along a line C-C′ in FIG. 14 together with the flow rate detecting element.
A flow rate detecting element 1 is constituted by a diaphragm portion 56a and 56b formed on two cavities 17a and 17b in a tabular semiconductor substrate 1a. A heat generating resistor (not shown) for detecting a flow rate is formed in the diaphragm portion 56a, and a fluid temperature detecting resistor (not shown) for detecting a temperature of a measured fluid is formed in the diaphragm portion 56b, respectively.
The flow rate detecting element 1 is accommodated within a concave portion 3 formed in a support body 2. The concave portion 3 of the support body 2 has surfaces having three stages of heights. The highest surface is constituted by surfaces 53a, 53b and 20, forms a surface defining the flow rate detecting element 1, and makes the surface of the flow rate detecting element 1 and the surface of the support body 2 approximately at the same height by setting a step (a distance to the surface of the support body 2) of the surface to a predetermined value. The surfaces 53a and 53b support a back surface of a leading end portion of the flow rate detecting element 1, and the back surface of the flow rate detecting element 1 and the support body 2 are fixed by an adhesive agent (not shown) in the surface 20.
The second stage of surface 21 is provided for forming a gap between a diaphragm portion 56a in which a heat generating resistor for detecting the flow rate of the flow rate detecting element 1 is formed and the support body 2.
Further, the third stage of surface 55 forms a deepest surface, is coupled to notches 54a and 54b for flowing an air, and is positioned in a lower surface of a diaphragm 56b in which the fluid temperature detecting resistor is formed. In accordance with the structure mentioned above, since the air flows into the diaphragm 56b in which the fluid temperature detecting resistor is formed, from the notches 54a and 54b by the surface 55, the fluid temperature can be detected at a high speed and a high precision.
However, in accordance with the conventional support structure, since the support body is structured in the three stages, and is formed in a complicated shape, a cost is increased.
Further, since the gap between the flow rate detecting element 1 and the support body 2 is formed by the second stage of surface 21, a part of the measure air flow flows to the back surface of the flow rate detecting element 1 from the gap. Accordingly, the measured air flow flowing to an upper surface of the diaphragm portion 56a in which the heat generating resistor for detecting the flow rate is formed is reduced and a measuring precision is deteriorated. This tendency noticeably appears particularly in a high flow rate region.
Further, in the case of being provided in the inflow air passage of the internal combustion engine of the motor vehicle or the like, a wide variety of dust, oil, water content and the like contained in the intake air affect. Reference numeral 19 shown in FIG. 15 denotes the wide variety of dust, oil, water content and the like contained in the air. In the conventional support structure, the third stage of surface 55 constructs a back chamber of the notches 54a and 54b corresponding to inlet and outlet of the air, and this portion triggers a reserve of the dust, oil, water content and the like 19, whereby a reliability is deteriorated. Further, since the gap exists between the flow rate detecting element 1 and the support body 2 in the second stage of surface 21, the reserve of the dust, oil, water content and the like 19 is generated in this gap, whereby a reliability is deteriorated.
Further, in the case that a high flow rate of air flows to the lower surface of the diaphragm 56b in which the fluid temperature detecting resistor is formed, there is a high risk that the dust 19 directly hit the diaphragm 56b so as to destroy as shown in FIG. 16, and a problem exists in a reliability.